This invention relates to railway track circuits.
Railway track circuits are used to detect the presence or position of railway vehicles in a length of railway track. A track circuit system typically comprises two rails of the railway track, side-by-side, each coupled near one end of a section of the track with a transmitter of electrical signals and near the other end with a receiver. When a vehicle is on the rails of the track circuit section of track its axles electrically connect the rails to each other and this is detected by the receiver as a change in the signal received from the transmitter. In a length of track there may be several such track circuit sections separated by, for example, insulative breaks in the rails, fixed connections between the rails ("shorting straps"), or electric filters having an inductance and/or capacitance which prevent the propagation of track circuit signals past the filter from one section of the track to another.
In such a system, if a break occurs in a rail then the transmitted electrical signals may leak through earth to the receiver, bypassing the track circuit and preventing a vehicle in the track section from being detected. If, because no vehicle is detected, it is assumed that there is no vehicle in the section and the section is therefore assumed to be safe for a second vehicle to enter then an accident could result. This is a problem with conventional track circuit systems.
To overcome this problem it is known for the receiver units of a track circuit to transmit data to an overall control system which can compare data from adjacent track circuit sections to ensure that a vehicle has entered one section of track when it is assumed to have left another section. However, this is inconvenient because it requires the control system to perform a function additional to its control function. It is preferable to have all vehicle detection functions performed outside the overall control system.
Also, in a conventional track circuit system the signal transmitted at one end of a track circuit section is attenuated as it passes through the rails to the receiver at the other end of that section (as illustrated in FIG. 1). Therefore, the voltage of the signal in the region of track near the receiver is relatively low. This causes problems because, unless the receiver is extremely sensitive, the change in the level of the received signal when a train connects together the rails in that region is particularly low and is not easily detected--especially if there is, for example, oil lying on the track which disrupts the connection between the rails made by the train. British Patent Specification No. 2 073 928 describes a track circuit system in which a receiver and a transmitter are provided at each end of a track circuit section, each receiver being capable of receiving signals from the transmitter at the opposite end of the track section to it. In determining the presence and/or position of any train in one track circuit section the system makes no use of the results of signalling in other track sections. Therefore, any determination of whether a train has moved from one track section to another must be performed by the system's overall control system.
Other examples of track circuit systems are described in U.S. Pat. Nos. 3 575 595, 3 829 682 and 4 498 650.